Circuit diagram design of redundant hot backup power supply

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When designing a high-reliability computer system, the supporting power supply is required to adopt a redundant design. In general, the solutions that can be adopted are capacity redundancy, redundant cold backup mode, N+1 backup mode of parallel current sharing, and redundant hot backup mode.
Capacity redundancy means that the maximum load capacity of the power supply is greater than the actual load, that is, the “large horse-drawn car”. The disadvantage is that it is not conducive to improving the efficiency of the power supply, and it has little significance for improving the reliability of the power supply.
The redundant cold backup mode means that the power supply consists of two or more unit modules with the same function. After the power is turned on, one of the unit modules supplies power to the device. When the working unit fails, the backup unit immediately starts to supply power to the device. The disadvantage of this method is that the startup of the backup unit to the output voltage requires a certain period of time, which is likely to cause a large gap in the output voltage, which will affect the equipment to be powered.
The N+1 backup mode of parallel current sharing means that the power supply is composed of multiple units with the same function. The sum of the output power of all units is greater than the power required by the system. The output of each unit is connected in parallel through the OR gate diodes, and sometimes the output is controlled by current sharing. Circuits, which are currently used more, are this way. N+1 backup mode Because multiple units supply power to the device at the same time, a single unit failure (failure) generally does not affect the output voltage, but if the output line fails, it will easily affect all units.
The redundant hot backup mode means that the power supply is composed of multiple units with the same function. When the power is started, all the units work at the same time, and the pre-set unit supplies power to the device. The backup unit is in an idle state, and the unit that supplies power to the device appears. In the event of a failure, the backup unit immediately supplies power to the device, maintaining the stability of the output voltage. The advantage of this method is that after the work unit is faulty, the backup unit output response speed is fast, and the output voltage can be guaranteed to fluctuate only within a small range.
This paper discusses in detail the power supply design scheme using redundant hot backup.

1 Working principle The main circuit of the redundant hot backup structure consists of two units with the same function and at the same time working. The switching circuit controls one of them to supply power to the device, and the other is unloaded. When a unit that supplies power to the device fails, the switching circuit acts immediately, causing another unit to supply power to the device while disconnecting the output of the failed unit.
The main circuit topology adopts a forward converter, which is composed of an input filter circuit, a power conversion circuit, a control circuit, an output filter circuit, and a monitoring switching circuit. The power supply block diagram is shown in Figure 1. The DC 28V input is filtered and supplied to the power conversion circuit. The control circuit controls the power conversion circuit through real-time detection to achieve an isolated and stable 5V voltage output, and overvoltage and overcurrent protection of the output voltage.

Figure 1 power block diagram

The redundant hot backup function is implemented by output monitoring and switch. In normal state, one of the two units supplies power to the device. When a fault occurs, another unit that is in hot backup immediately supplies power to the device and cuts off the output of the failed unit. If the output monitoring circuits of the two units fail at the same time, the two units supply power to the device at the same time. Since the output terminals of each unit are connected with the OR gate diode, this is the backup of the power supply output in parallel mode.

2 Circuit implementation of monitoring and switching function As a redundant hot backup power supply, the main problem is the fault diagnosis of the working unit. If the sensor is set for each possible fault point of the power supply, the fault is judged by the smart chip or the discrete chipset, and then the corresponding switching control is adopted, the complexity of the entire power supply is increased, and the reliability of the fault detection judgment part is not necessarily high. On the power supply itself. Since the main fault of the power supply is reflected in the output voltage, it is used as a criterion for fault judgment to monitor whether the output voltage of the working unit is within the set range. The circuit for monitoring and switching functions is shown in Figure 2.

Figure 2 Output monitoring, switching circuit schematic

In Figure 2, R18, R19, V10, D1 constitute the 5V output switch of unit 1, D1 is turned on, the 5V output of unit 1 is cut off; R018, R019, V010, D01 constitutes the 5V output switch of unit 2, its function and Unit 1 is the same. R34, R35, R33, AJ4 (TL431), R20*, R22, C20, R21*, AJ2 (TL431), D2, R24, R23, C21, V13, R30 realize the output voltage monitoring and control unit 2 output switching of unit 1. Switch function: When the output voltage of unit 1 is higher or lower than the set voltage range (adjust the resistance value of R34, R35, R20*, R21* can change the set voltage range), pin 1 of optocoupler D2 2, no current flows, D2 feet 4, 5 are cut off, V13 base voltage becomes low, D01 feet 1, 2 do not flow current, so that the switch of unit 2 is turned on, unit 2 outputs voltage to the device. At the same time, when the output voltage of the unit 2 is within the set voltage range (the resistance values ​​of the R034, R035, R020*, and R021* can be adjusted to change the set voltage range), the currents of the pins 1 and 2 of the optocoupler D02 flow. The pins 4 and 5 of D02 are turned on, the voltage of the base of V013 becomes high, and the currents of the pins 1 and 2 of D01 flow, so that the switch of the unit 1 is turned off, and the unit 1 does not output a voltage to the device. Similarly, the components in the symmetrical position of the unit 2 realize the same output voltage monitoring and control unit 1 outputting the function of the switching switch as in the unit 1. By setting the capacitance values ​​of C21 and C021 to set which unit supplies power to the device at the start of power supply, the feet 4 and 5 of D2 and D02 will have extremely short conduction during power-on, and the capacitance values ​​of C21 and C021 are small. The unit first cuts off the path that another unit supplies power to the device. It can be seen from Fig. 2 that in the two units of the power supply, if the unit 1 outputs the correct voltage first, the output of the unit 2 is turned off; the output voltage of the unit 1 is incorrect, and the output switching switch of the unit 2 is turned on to supply power to the device. At the same time, the output voltage of the unit 1 is cut off. vice versa.

3 Experimental results Observing the output of the two units with an oscilloscope, it can be observed that unit 1 outputs voltage to the device, unit 2 does not output voltage to the device; disconnecting the input voltage of unit 1 can observe that the output voltage has no change, by unit 2 Power the device. If the input voltage is first supplied to the unit 2, then the input voltage is supplied to the unit 1, and then the input voltage of the unit 2 is turned off, the same is true.
Figure 3 shows the waveform of the backup unit immediately supplying power to the device when the working unit fails due to a fault or voltage drop. It can be seen that the output voltage returns to the standard value within 10ms and does not cause the computer to restart.

Figure 3 switching waveforms

4 Summary This paper presents a method to achieve redundant hot backup power supply, the circuit is simple, which is conducive to improve the overall reliability of the power supply. However, in high-power applications, the switch should be carefully selected to reduce its effect on the output voltage, and the output voltage multi-point feedback method is considered to compensate for the effect of the switch on the output voltage.